Special Nuclear Materials (SNM) and radioactive sources, many of which emit neutrons, are a concern for a wide variety of organizations and programs, from Homeland Security to health physics to nuclear power. The ability to detect such materials in the field, through the use of neutron detectors, is thus a practical and pressing need.
Neutron dosimeter “balls” are readily available commercially, but these can only provide integrated dose rates, and are not helpful in locating the source of the neutron flux. The ability to determine the direction of a neutron source, using a simple device based on robust and inexpensive technology, would therefore be useful to a wide range of disciplines.
Previous attempts at directional neutron detectors have been unable to address issues inherent with providing directional neutron detection in the field. For example, detectors have been developed for well-logging in the oil and gas industry. These are based on detection of neutron and boron reaction products via ionized gas, and the directionality is provided solely by directional shielding. While the system is robust, given its use in oil wells, it is also only sensitive to thermal or epithermal neutrons.
Another directional neutron detector is based on gas detection of boron+n reaction products, this time in a large gas volume, where directionality is determined by reconstruction of the tracks the reaction products make through the gas, which is a position sensitive ionization chamber. This is severely rate limited.
Other detectors are based on a large array of scintillating fibers coupled to photosensitive detectors. Directionality of neutrons is estimated from the sequence of fibers traversed by the scattered protons and energy deposited in each one of them. In another case, the scintillation fibers are variously low wavelength emitters or higher-wavelength emitters; neutrons coming in the preferred direction create an “anticorrelation” in two photo multiplier tubes (PMTs) at the two wavelengths, while neutrons from perpendicular direction hit multiple fibers and cause “correlation” in the two PMTs. Gamma events are also discriminated by vetoing correlated PMT signals, as the fibers are not inherently neutron-gamma discriminating.
Other designs are also based on scintillating fibers, but use sophisticated computer algorithms that look at differences in the signals from different angles, and are only sensitive to high energy neutrons.
Other detectors use 3He tubes, which is too rare and expensive for widespread use. Other detectors use large area silicon detectors, so they are also too expensive for widespread deployment in the field. Other examples use a large gas ionization chamber that is filled with 4He for high energy neutrons and 3He for thermal neutrons, with a camera to record the tracks of recoils and determine directionality. This is severely rate limited, requires a large volume of gas, and requires the use of 3He.
Other detectors are based on direct detection of the recoiling proton from neutron+hydrogen reactions in thin hydrogenous material (where the hydrogenous material is not a scintillating material). The recoiling proton is sent through a collimation mechanism to be detected in a charged particle detector at the end, such that directionality is achieved as protons from perpendicularly incident neutrons do not make it through the collimator. The use of charged particle detection is difficult in the field as it requires vacuum.